1. Field of the Invention
The present invention relates to an antilock brake control system (ABS), and more particularly, to antilock brake control system equipped with a function for learning and correcting the condition for starting the brake pressure suppression.
2. Description of the Prior Art
An antilock brake control system overrides the driver's operation of the brakes when the brakes ar applied and the tires lock and begin to slip on the road surface to automatically adjust the fluid pressure in the brake cylinder to stop the wheel locking and control the slipping of the tire to the road surface so that the maximum coefficient of friction .mu. is obtained therebetween.
Conventional ABS systems define a constant slip threshold and are configured to begin brake pressure suppression based on the single condition of the actual slipping of the tire exceeding this slip threshold.
As shown in FIG. 5 , however, because the relationship between the coefficient of friction .mu. and the slip characteristics of the road surface is varied, the degree of tire slipping at which the coefficient of friction .mu. is greatest differs according to the road surface, and the wheel behavior after the coefficient of friction .mu. exceeds this maximum slippage is also varied. For example, while on road surface R1 a relatively constant coefficient of friction .mu. is maintained after the maximum value for that road is exceeded, the coefficient of friction .mu. decreases slightly and greatly for road surfaces R2 and R3, respectively.
However, because the starting point for brake pressure suppression is determined according to a constant slip threshold Tx in a conventional antilock brake control system, antilock brake control on any road surface is a repetition of steps such that: the suppression of brake pressure starts at point A while tire slipping increases to the "return point" B, after which the locking symptoms are eliminated and tire slipping returns to the level at point C as shown in FIG. 5. On road surface R1, because the starting point A for brake pressure suppression is slightly before the point of maximum coefficient of friction .mu., antilock brake control tends to start early, thus preventing attainment of the greatest coefficient of friction .mu. for that road surface. On road surface R3, however, because tire slipping increases greatly from the pressure suppression starting point A to the return point B, the maximum time and distance are required for the locking symptoms to be eliminated. It is on road surface R2 that the best antilock brake control is obtained relative to the road surface.
Therefore, an object of the present invention is to provide an antilock brake control device whereby the start of 5 brake pressure suppression can start under continuously ideal conditions even when the condition of the road surface changes. A further object is to provide an antilock brake control device which can, for example, set a slip threshold Tx' for a road surface R1, and set a slip threshold Tx" for a road surface R3.